Hypokeimenon (Greek: ὑποκείμενον), later often material substratum, is a term in metaphysics which literally means the "underlying thing" (Latin: subiectum).
[1] The existence of a material substratum was posited by John Locke, with conceptual similarities to Baruch Spinoza's substance and Immanuel Kant's concept of the noumenon (in The Critique of Pure Reason).
Locke saw this ontological ingredient as necessary if one is to be able to consider objects as existing independently of one's own mind.
The material substratum proved a difficult idea for Locke as by its very nature its existence could not be directly proven in the manner endorsed by empiricists (i.e., proof by exhibition in experience).
[2] Nevertheless, he believed that the philosophical reasons for it were strong enough for its existence to be considered proved.